Three way catalysts are used in internal combustion engine exhaust systems, e.g., automotive exhaust systems, to convert the carbon monoxide, hydrocarbons and nitrogen oxides present in automotive exhaust gases into more desirable components. A wide variety of catalysts have been used including rhodium carried on an alumina support material. Rhodium is particularly effective for controlling emissions of nitrogen oxides (i.e. NO.sub.x) and is generally used in combination with other noble metals (e.g., Pt or Pd) to effect the full range of exhaust pollutant control. In practice, rhodium is often segregated from the other noble metals in the same catalyst by dispersing it in a separate washcoat layer or within the same washcoat layer but upon a different support phase. One of the drawbacks of such a catalyst is that it has a tendency to become deactivated at high temperatures particularly in oxidizing gas mixtures. Rhodium interacts strongly with gamma-alumina at high temperatures above 500.degree. C. but .alpha.-alumina was found to interact only weakly with rhodium after oxidizing treatments up to 900.degree. C. as disclosed in U.S. Pat. No. 4,172,047, which patent is commonly assigned with the present invention.
Other problems related to the deactivation of rhodium and other noble metals are 1) loss of alumina surface area and associated occlusion of noble metal particles, and 2) sintering or coalescence of small noble metal particles into larger particles. It would be desirable to further minimize the interactions between rhodium and alumina, while simultaneously decreasing the extent of rhodium occlusion and sintering. The present invention has been found to meet these objectives.